1. Field of the Invention
The present invention relates to an L-band optical amplifier for amplifying optical signals with the wavelength band of the L-band using an erbium-doped fiber. More particularly, the present invention is directed to an L-band optical amplifier for amplifying the L-band wavelength by pumping an erbium-doped fiber by means of spontaneous emission.
2. Description of the Related Art
Optical amplifiers have been typically used in an optical communication system to compensate optical signal loss from a long-distance transmission. In particular, erbium-doped fiber amplifiers (EDFAs) have been widely used owing to its excellent amplification efficiency and low noise generation. For example, the wavelength band of the C-band ranging from 1530 nm to 1565 nm is generally used.
FIG. 1 shows a conventional C-band optical amplifier for amplifying optical signals in the wavelength band of the C-band. As shown in FIG. 1, the C-band optical amplifier includes a first pumping light source 130, a second pumping light source 160, a first wavelength selective coupler 120, a second wavelength selective coupler 150, a first isolator 110, a second isolator 170, and an erbium-doped fiber 140.
Each of the first and second pumping light sources 130 and 160 may be constructed by using a laser diode, which outputs light at a wavelength of 980 nm or 1480 nm. In order to pump the erbium-doped fiber 140, the first pumping light source 130 outputs the first pump light in a forward direction and the second pumping light source 160 outputs the second pump light in a backward direction.
The first wavelength selective coupler 120 is interposed between the first isolator 110 and the erbium-doped fiber 140 and inputs the first pump light and the optical signals inputted from the first isolator 110 into the erbium-doped fiber 140.
The second wavelength selective coupler 150 is interposed between the second isolator 170 and the erbium-doped fiber 140, inputs the second pump light into the erbium-doped fiber 140, and outputs the optical signals amplified at the erbium-doped fiber 140 to the second isolator 170.
The erbium-doped fiber 140 is interposed between the first wavelength selective coupler 120 and the second wavelength selective coupler 150 and is pumped by the first and second pump light outputted from the first and second pumping light sources 130 and 160 to amplify optical signals belonging to the C-band.
The first isolator 110 serves to minimize a loss caused by optical signals traveling in a reverse direction. The first isolator 110 allows optical signals belonging to the C-band received from an exterior source to pass through but prevents optical signals inputted from the first wavelength selective coupler 120 to pass through in a reverse direction.
The second isolator 170 transmits optical signals outputted from the second wavelength selective coupler 150 but prevents backward optical signals inputted from an exterior source back to the inside of the C-band optical amplifier.
In order to effectively cope with a recent rapid growth in communication demand, various approaches for using a plurality of wavelength bands, such as the C-band between 1530 nm and 1565 nm, the L-band between 1565 nm and 1610 nm, and the S-band between 1450 nm to 1510 nm are being developed.
FIG. 2 shows a configuration of an L-band optical amplifier using a pumping light source with the wavelength band of the C-band according to the prior art. As shown in FIG. 2, the L-band optical amplifier includes a first isolator 210 and a second isolator 250, an erbium-doped fiber 240, a wavelength selective coupler 220, and a pumping light source 230. Note that the L-band optical amplifier amplifies optical signals with the wavelength band of the L-band by means of pump light with the wavelength band of the C-band.
The first isolator 210 causes optical signals with the wavelength band of the L-band received from an exterior source to be forwarded to the wavelength selective coupler 220 but prevents backward optical signals from the wavelength selective coupler 220.
The pumping light source 230 may be constructed by using a laser diode generating light at a wavelength of 1550 nm. The pumping light source 230 emits pump light belonging to the wavelength band of the C-band to pump the erbium-doped fiber 240.
The erbium-doped fiber 240 is pumped by the pump light and amplifies the optical signals by stimulated emission of a wavelength of light, such as inputted optical signals with the wavelength band of the L-band.
The wavelength selective coupler 220 is interposed between the erbium-doped fiber 240 and the second isolator 250 and serves to input the pump light and the optical signals with the wavelength band of the L-band into the erbium-doped fiber 240.
The second isolator 250 causes the optical signals inputted from the erbium-doped fiber 240 to be forwarded to the outside of the L-band optical amplifier but prevents optical signals inputted from the exterior source back to the L-band optical amplifier.
FIG. 3 shows a configuration of a conventional L-band optical amplifier using a C-band optical amplifier as a pumping light source. As shown in FIG. 3, the L-band optical amplifier includes a C-band optical amplifier 330, a wavelength selective coupler 320, a first erbium-doped fiber 340, first and second isolators 310 and 350. Note that the C-band optical amplifier 330 functions as a pumping light source of the L-band optical amplifier.
The C-band optical amplifier 330 includes a C-band light source 331, first and second pumping light sources 334 and 337, a second erbium-doped fiber 335, an tunable filter 338, third and fourth isolators 332 and 339, second and third wavelength selective couplers 333 and 336. As shown, the C-band optical amplifier 330 outputs pump light with the wavelength band of the C-band so as to pump the L-band optical amplifier.
The C-band optical amplifier 330 has the same configuration as that of the C-band optical amplifier shown in FIG. 1. For this reason, the detailed description will be omitted. However, unlike the C-band optical amplifier shown in FIG. 1, the C-band optical amplifier 330 further comprises the light source 331 with the wavelength band of the C-band, thus the C-band optical amplifier 330 functions as the pumping light source for the L-band optical amplifier.
The first wavelength selective coupler 320 allows the pump light of the C-band inputted from the C-band optical amplifier 330 and the optical signals of the L-band inputted from the first isolator 310 to be forwarded into the first erbium-doped fiber 340. The first erbium-doped fiber 340 is pumped by the C-band pump light to amplify optical signals with the wavelength band of the L-band.
The first isolator 310 allows optical signals received from an exterior to be inputted into the first wavelength selective coupler 320 but prevents the backward optical signals inputted from the first wavelength selective coupler 320. Similarly, the second isolator 350 blocks the optical signals traveling in a reverse direction, thus minimizing a loss resulting from the optical signals traveling in a reverse direction.
However, the erbium-doped fiber generally has a lower absorption and emission value at the wavelength band of the L-band, as compared with that at the wavelength band of the C-band. Therefore, when the L-band optical amplifier makes use of a laser diode or a C-band optical amplifier, which has the wavelength band of the C-band, as a pumping light source, the L-band optical amplifier has difficulty in obtaining a higher output power and generating a greater noise figure. Such L-band optical amplifier also tends to increase the production cost. In particular, when the L-band optical amplifier makes use of a C-band optical amplifier as a pumping light source, the L-band optical amplifier must provide a separate light source. This type of arrangement further increases the volume of the amplifier and yields a complicated construction.